This invention relates to an optical transmission module and its manufacturing method. More particularly, the invention relates to an optical transmission module and its manufacturing method facilitating reliable fixture of guide pins for an optical connector in the optical transmission module and improving the performance of the optical transmission module.
In the recent technical field of optical communication, there is a demand for techniques for transmitting a huge amount of data at a high speed in practical applications such as data transfer between LANs (local area networks) or between boards in a computer system. A typical transmission system used for data communication consists an optical transmitter module, an optical transmission medium and an optical receiver module. The optical transmitter module includes a laser diode, control IC and optical coupling elements for connection to an optical connector. The optical transmission medium includes the optical connectors having means for connection to respective modules and an optical fiber. The optical receiver module includes a photodiode, control IC and optical coupling elements for connection to the optical connector. In most cases, guide pins are used for connection between the optical connector and individual modules. In the present specification, the optical transmitter module and the optical receiver module, among these components, are collectively called "optical transmission modules", or sometimes simply "modules".
Optical transmission modules are desired to be compact, light, highly durable and low in power consumption. On the other hand, there is the need for development of modules including multi-channeled optical fibers, high-speed optical semiconductor element and control IC, aiming a greater amount of data and a higher data transfer rate. However, a further sophisticated structure is required to realize a module having a increased number of the transmission channel. That is, a simple and reliable structure must be proposed while keeping the heat dissipation high enough. A high productivity and a high production yield are also required in order to supply the module at a low price.
FIG. 14 is a schematic plan view of a central part of a optical transmission module made by the Inventor on an experimental basis in the process to accomplishment of the present invention. The optical transmission module includes an optical semiconductor element 102a and a signal processing IC 102b disposed on a base 101 with a predetermined configuration. The base 101 is a substrate made from a silicon wafer, for example. The optical semiconductor element 102a is a semiconductor element such as laser diode or photodiode. Located in front of the optical semiconductor element 102a is one end of an optical fiber 103 to input or output an optical signal. The optical fiber 103 is fixed by an optical fiber holder 104 as a reinforcement element. Connected to the other end of the optical fiber 103 is an optical connector, not shown, so that it can exchange optical signals with other optical transmission modules via optical fibers, not shown. In order to ensure optical coupling with the optical connector, not shown, guide pins 105, 105 are provided.
Although there is shown the case having only one optical fiber 103, discussion made here is applicable also to an element having a plurality of optical fibers 103 aligned in parallel and a plurality of laser diodes or photo diodes disposed in an array as optical semiconductor elements 102a. For such multi-channeled optical transmission modules, a rapid increase of demand is expected in applications such as inter-board connection.
The silicon base 101 has formed guide pin fixing grooves 101a. These grooves 101a have a V-shaped cross-sectional configuration which can be made by anisotropic wet etching utilizing a crystallographic anisotropy of silicon. The guide pins 105 are held in the guide pin fixing grooves 101a with their ends extended beyond the silicon base 101 for engagement with the optical connector, not shown, and are secured on the base 101 by an adhesive 106.
In order to ensure optical coupling of the optical fiber 103 of the module to an optical fiber of the optical connector, the base 101 and the optical connector must be brought into close contact.
However, close contact between the module and the optical connector invites a problem caused by heat generation of the element and a problem regarding its assembly. These problems are discussed below in greater detail.
The problem of heat generation of the element is discussed first.
In the optical transmission module shown in FIG. 14, heat generated from the optical semiconductor element 102a and IC 102b is released through a heat sink (not shown) in contact with the bottom plate of the base 101. That is, heat generated by these elements is externally released from the base 101 through a heat sink (not shown).
IC 102b typically has a circuit arrangement whose power consumption per one channel is 0.2 to 0.3W. Therefore, power consumption is within 0.8 to 1.2W in a structure for four channels, but is large as 2 to 3W in a structure for ten channels. According to experiments by the Inventor, in such modules, the temperature at one end 101c of the base 101 sometimes exceeded 70.degree. C. during the operation, depending on the mode of heat dissipation and the ambient temperature. Since the maximum allowable temperature of the optical connector connected to the module was lower than 70.degree. C. in most cases, the module often failed in normal data transfer due to an optical misalignment caused by heat from the module.
The Inventor made investigation on heat movement paths and found that heat conduction through guide pins 105 was large. That is, since the guide pins 105 are made of a metal with a high thermal conductivity, the optical connector is heated through them. Once the optical connector is heated, a thermal strain occurs, and the optical alignment of the optical connector deviates from the optical alignment of the module.
A multi-mode fiber has the core diameter of 50 .mu.m giving a certain extent of allowance. A single mode fiber for single mode transmission, however, has the core diameter as small as approximately 10 .mu.m, a smaller optical misalignment, as small as several .mu.m, results in increasing the optical coupling loss, and disables stable data transfer.
On the other hand, for close contact between the module and the optical connector, there was also a problem on their assembly. That is, an end surface 101c of the base 101 had to be planar, and the adhesive had to be applied carefully not to overhang onto the end surface 101c.
If the adhesive 106, before setting, flows through gaps between guide pins 105 and the guide pin fixing grooves 101a by a capillary action onto the end surface 101c of the base 101, the optical connector cannot get into close contact with the optical module. To prevent the problem, it is necessary to make means for preventing extrusion of the adhesive. As one of such means, the Inventor experimentally made grooves 101e along the guide pin fixing grooves 101a to prevent the extrusion of the adhesive.
The extrusion preventing grooves 101e can be made by etching similarly to the guide pin fixing grooves 101a. However, an uniform stirring of an etchant is disturbed at the grooves 101e in the etching process. It results in changing the etching rate at the guide pin fixing grooves 101a from the etching rate at the remainder part and in difficulty to make guide pin fixing grooves 101a with a uniform configuration. Therefore, the use of the extrusion preventing grooves 101e involved the problem of a low production yield of the base 101.
On the other hand, it is technically difficult to first make the grooves 101a, then apply a photo resist onto the upper surface of the base 101 and slope surfaces of the grooves 101a, and make photo resist apertures to be used for etching the extrusion preventing grooves 101e by PEP (photo-engraving process). In other words, it is difficult to make guide pin fixing grooves 101a and overflow preventing grooves 101e separately by the anisotropic wet etching.
It would be also possible to make extrusion preventing grooves 101e by using a dicing machine after the guide pin fixing grooves 101a are made. However, chipping is liable to occur at corners where the extrusion preventing grooves 101e cross the guide pin fixing grooves 101a, and it is difficult to prevent the chipping. Here again, therefore, the problem of a low production yield of the base 101 remains.
It is necessary to make a metal thin film wiring pattern on the base 101 for electric connention of electronic elements including optical semiconductor elements. The surface of the metal thin film wiring pattern had better be clean for subsequent wire bonding or mounting of the electronic elements. For this purpose, the step for making the metal thin film wiring pattern had better be the final step in the manufacturing process of the base 101. The metal thin film wiring pattern is typically made by the so-called lift-off process. Namely, by applying a photo resist on the entire upper surface of the base 101, making a photo resist opening in a location for the metal thin film wring pattern by PEP, and stacking a metal thin film thereon by vapor deposition, for example. Thereafter, when the photo resist is removed by using an organic solvent, the metal thin film on the photo resist is also removed together, leaving the metal thin film wiring pattern of a predetermined configuration on the base 101.
However, grooves on the base 101 causes the photo resist applied thereon to be uneven in thickness. Particularly, the extrusion preventing grooves 101e additionally formed on the base make the irregularity of the base surface more complicated, and make the unevenness in thickness of the photo resist larger. Here again, the problem of a low production yield of the base remains.
For assembling such modules, there is often used the method configured to fix guide pins 105 in guide pin fixing grooves 101a with an adhesive 106 applied in gaps between the bottom surfaces of the guide pin fixing grooves 101a and the guide pins 105 placed therein. In this method, the adhesive 106 results in being applied under the guide pins 105 from one end of each guide pin 105 nearer to the optical semiconductor element. Usually, the adhesive is applied from a nozzle. In order to prevent overflow of the adhesive from the opening at the ends of the guide pins 105 nearer to the optical semiconductor element, injecting conditions of the adhesive, such as injecting pressure and injecting speed, must be determined appropriately. However, since these injecting conditions involves various factors including size and the position of the injecting nozzle, the viscosity of the adhesive, injecting pressure and injecting time, it is difficult to control the quantity of the supplied adhesive, which results in decreasing the production yield in terms of fixture of the guide pins.